Since the invention of meeting minutes people have been having quiet off-the-record conversations in the hallway before the meeting... WhatsApp is just that in digital form.
If you think this isn't the case I assume you're not a senior decision-maker.
Confidential channels where you can brainstorm ideas that aren't politically palatable and you feel safe to express your true opinions and concerns are critical.
There are so many issues that the COVID inquiry should address. Like whether lockdown should be part of our pandemic strategy going forward and the long-term effects on all parts of society. This WhatsApp crap is a stupid irrelevant distraction.
I judge Boris harshly for his COVID/lockdown actions, I'm not defending him. But I don't need to see his private conversations during a highly uncertain time to make that decision.